Transmission Swaps & Modifications

Transmission Options for the Nailhead

Created By: Russ Martin at

There are several transmission choices for the Nailhead engines, I will start with the best and easiest to use. The 64-65-66 401-425 came with an excellent automatic trans, the ST400. They have a round shaped bell housing unlike other GM transmissions. This trans is super strong and the 65-66 version had a two speed (switch pitch)torque convertor. The stall is changed in the convertor with an electric solenoid so you can change the stall speed (RPM will raise gearing the trans down) with a flip of a switch! I like using the smaller 64-67 ST300 convertor in lighter cars for even a higher stall RPM. I have a picture of a wiring diagram posted to wire for best performance. THE SWITCH AND STANDARD CONVERTORS AND FRONT PUMPS ARE DIFFERENT so don't mix them up.

The 64 is better than the Dynaflow but not as a good as the 65-66 ST400 (available in 67 also but has the BOP pattern and will not bolt to the Nailhead). The 64 trans case and valve body are one year only so no shift kits are available and the convertor is the standard GM type. This trans can be identified by the single wire connecter (switch pitch has two) at the rear and a weld on shift lever (the 65 and later bolt on). The 64 also is lacking in controlling down shifts, it shift 3 times but only has a low and drive. If you want to hold it in each gear you start off in L, wind it up and shift to D, that puts it in second gear, now pull it back into L and that holds it in second, wind it up again and shift to D and you are in third. Here is the part I don't like, say you are driving in the mountains, the turns are tighter and slower, you drop in down to L holding it in second gear, then you have to slow down to about 25 mph to make a tighter turn and bam! it goes into low, then you faster, put it into D to put it back in second and and back to low to hold it there.WHEW! I drove a 64 Riviera everyday for 13 years so I know. OK, now you want to know if the 64-66 trans will bolt to earlier Nailheads right? you can bolt this trans on to all Nailheads from 57-63 with my special crank adapter bushing and the correct flex plate, the 59-63 cal use the 401-425 flexplate but because ALL Buicks are externally balanced and different flexplate balanced to the 364 spec's must be used. The 57-63 starters (yes 4 different ones used) have a 1/2" longer snout for the Dynaflow so the starter from the 64-66 (or my mini gear reduction starter) must be used and will bolt onto the 61-66 blocks. The 57-60 364-401's will need one of special starters with a different mounting bolt pattern. Another option is the adapters we sell to mount a Chevy trans behind the Nailhead and we sell them if that is what you want. BEWARE of venders selling UNBALANCED flex plates and flywheels for less money, you will have to pay as much as 150.00 to balance the flywheel it will cost more! The 53-56 264/322 has very few options, the bell housing and flywheels look the same as the 57-66 but in fact they are about 1" larger. We sell an adapter for the Chevy 350-400-700R4-2004R and it is possible to adapt a Ford C-4 with a flat-o-matic adapter bolted to a flathead Ford trans adapter. A custom convertor will have to be built but it is a very neat and tight package when it is done and looks like like it was made for it.

Getting rid of the torque tube

If you have a 1955 and earlier Buick, bad news, the original rear end has to go, replace it with a 1972-77 GM A body 10 bolt rearend, not a Chevy 10 bolt, those have crappy axle bearings and are weak. You can also swap in a Ford 9", many company's will make one up for you that is all new. If you have a 1956-1960 Buick you are in luck, a 1961-62 center section that has an open drive will replace your torque tube center section in your rearend housing. You can make a 4 bar setup BEFORE taking the rearend apart or go the easy way and buy the Torque Tube Eliminator Kit I have in my Drive Line Parts section. Now any late model transmission can be used in your Buick.

Stick Shift Transmission Options

Very few Buicks came from the factory with manual transmissions, maybe 1-2% so finding original bell housings and flywheels are very hard and many different ones were used, some need special clutches. The 57-63 and 64-66 364-401-425 are covered, we have new BALANCED steel flywheels and cast aluminum bell housings. Our flywheels are new steel billet, much safer than an unknown cast iron original that could end up being a BOMB! They are drilled for an 11 diaphragm clutch, soft and easy on your leg. Most GM 3-4-5 speeds bolt to this new heavy cast aluminum bell housing. With the 53-56 264/322 we are not so lucky, we have to find used flywheels and bell housings and to make it worse there are 3 different flywheels. The 53-55 322, the 54-55 264 and the 56. As with the 364 and 401 the 264-322 have different flywheel balance so you will need to contact me to ID yours. It is possible to use a Ford flathead trans adapter, bolt it to your 264/322 and by the later GM trans to Ford flathead adapter from Speedway Motors bolt them together and a 3-4-5 speed with bolt to the early Nailhead.